Cell Signaling and Chemotaxis

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Cell Signaling and Chemotaxis
Read Chapter 15 of Molecular Biology of the
Cell
Example for cell signaling in unicellular
organisms chemotaxis in bacteria (move cell
optimally in environment), sexual mating in yeast
(coordinate conjugation into cell with new
assortment of genes)
Signaling cell releases signaling molecules S,
target cell responds by means of receptors
(usually on cell surface) that bind S and
initiate a response in the target cell in
chemotaxis S is an environmental factor
2
Sexual Mating in Yeast (coordinate conjugation
into cell with new assortment of genes)
When a haploid individual is ready to mate, it
releases a peptide mating factor that signals
cells of the opposite mating types to stop
proliferating and prepare to conjugate the
subsequent fusion of two haploid cells of the
opposite mating types produces two diploid cells,
which then undergo meiosis and sporulate to
generate haploid cells with a new assortment of
genes (Alberts et al, Chpt. 15)
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Cell Signaling and Chemotaxis
Read Chapter 15 of Molecular Biology of the
Cell
Example for cell signaling in unicellular
organisms chemotaxis in bacteria (move cell
optimally in environment), sexual mating in yeast
(coordinate conjugation into cell with new
assortment of genes)
Signaling cell releases signaling molecules S,
target cell responds by means of receptors
(usually on cell surface) that bind S and
initiate a response in the target cell in
chemotaxis S is an environmental factor
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Action of Hormone Receptors
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Action of Hormone Receptors
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G-proteins are Signal Transducers
MissingG-protein
signal
signal
Receptor
Amplifier
G-protein
Receptor
Amplifier
Cell membrane
Cytosol
No Biological effect
Biological effect
Malfunctioning G-proteins disturb the
intracellular signaling pathways, altering normal
cell functions.
G-proteins transmit and modulate signals in
cells. They can activate different cellular
amplifier systems.

GTP ? GDP Pi 7.3 kcal/mol
Ras
switch I

• smallest G-protein (189 residues, 21KDa mass)
• acts as a molecular switch
• cycles between an active (GTP-bound) and an
  inactive (GDP-bound) state
• major conformational changes during the
  signaling cycle take place in the switch I and
  switch II regions
• switching activity regulated by GAP and GEF
  proteins
• activated forms of Ras genes are found in 30 of
  human tumors.

switch II
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Signaling Cycle of Ras
signal IN
OFF
R-state
GTP hydrolysis is induced by GAP protein
Pi
GTP hydrolysis
Guanine nucleotide Exchange Factor
GTPase Activating Protein
GEF
Conformationalchange
GAP
GDP
Exchange of GDP for GTP is catalyzed by GEF
protein
T-state
ON
signal OUT
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Mechanical Cycle of Ras/Spring
?
Ras
?
Ras/GTP hydrolysis, induced by GAP, leads to
Ras/GDP in T-state
GTP hydrolysis
GTP
Ras/GDP
Pi
?
?
Ras/GDP evolves irreversibly and spontaneously
from T-state to R-state
GAP
Ras
Ras/GDP
GDP
T-state
?
?
? 1ns
Ras separates from GAP, then exchanges GDP for
GTP, and the reverse R-to-T transition takes place
GAP
T?R transiton
Ras
GDP
R-state
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What Happens After GTP Hydrolysis?
Switch I
RAS/GDP
RAS/GTP
strong fluctuations R-state
small fluctuations T-state
Switch II
Switch II helix melts altering the contact area
of RAS!
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The Role of Modules in Signaling
John D. Scott and Tony Pawson Scientific American
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Scaffolds Speed Signal Transmission
John D. Scott and Tony Pawson Scientific American
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Neutrophils are our body's first line of defense
against bacterial infections. After leaving
nearby blood vessels, these cells recognize
chemicals produced by bacteria in a cut or
scratch and migrate "toward the smell". The above
neutrophils were placed in a gradient of fMLP (n
formyl methionine- leucine- phenylalanine), a
peptide chain produced by some bacteria. The
cells charge out like a "posse" after the bad
guys. http//www.cellsalive.com/chemotx.htm
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Phagocytosisn Cell Eats Cell
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Chemotaxis of neutrophil chasing a
bacterium (http//www.hopkinsmedicine.org/cellbio/
devreotes/movies.html)
This video is taken from a 16mm movie made in the
1950s by the late David Rogers at Vanderbilt
University. It was given to me via Dr. Viktor
Najjar, Professor Emeritus at Tufts University
Medical School and a former colleague of Rogers.
It depicts a human polymorphonuclear leukocyte
(neutrophil) on a blood film, crawling among
red blood cells, notable for their dark color and
principally spherical shape. The neutrophil is
"chasing" Staphylococcus aureus microorganisms,
added to the film. The chemoattractant derived
from the microbe is unclear, but may be
complement fragment C5a, generated by the
interaction of antibodies in the blood serum with
the complement cascade. Blood platelets adherent
to the underlying glass are also visible.
Notable is the characteristic asymmetric shape of
the crawling neutrophils with an
organelle-excluding leading lamella and a
narrowing at the opposite end culminating in a
"tail" that the cell appears to drag along.
Contraction waves are visible along the surface
of the moving cell as it moves forward in a
gliding fashion. As the neutrophil relentlessly
pursues the microbe it ignores the red cells and
platelets. However, its leading edge is
sufficiently stiff (elastic) to deform and
displace the red cells it bumps into. The
internal contents of the neutrophil also move,
and granule motion is particularly dynamic near
the leading edge. These granules only approach
the cell surface membrane when the cell changes
direction and redistributes its peripheral "gel."
After the neutrophil has engulfed the bacterium,
note that the cell's movements become somewhat
more jerky, and that it begins to extend more
spherical surface projections. These bleb-like
protruberances resemble the blebs that form
constitutively in the M2 melanoma cells missing
the actin filament crosslinking protein filamin-1
(ABP-280) and may be telling us
something about the mechanism of
membrane protrusion.
Written by Tom Stossel, June 22,
1999.
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Bacterial Motility Typical of Flagellated Bacteria
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Explain Tumbling Mechanism
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Signaling and Adaptation in Chemotaxis
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Receptor Signaling Complexes in Chemotaxis
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Genetics of Chemotactic Signaling System
http//www.genome.ad.jp/kegg/pathway/eco/eco02030.
html
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Show html
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Adaptation in Chemotaxis
slow
slow
intermediate
fast
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End
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Signaling and Adaptation in Chemotaxis
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Summary of the experiments of Lumsden and Davies
showing chemotaxis between neural tissue
(trigeminal ganglion) and its target (whisker
pad). The chemoattraction is specific for (A) the
target and (B) the epithelial cells of the
target. Moreover, the chemotactic ability of the
whisker pad is specific for the trigeminal
neurons.